Grinding high-temperature alloys



Mai-ch 11', 1969' n'sLHA- N 3,431,685

GRINDING HIGH-TEMPERATURE ALLOYS I Filed Feb. 15, 1965 Sheet INVENTOR.

March 11, 1969' R. s. HAHN 3,431,685

GRINDING HIGH-TEMPERATURE ALLOYS Filed Feb. 15. 1965 Sheet. 2 of 11INVENTOR. Robert .5. Hahn March 11, 1969 Filed Feb. 15. 1965 WHEEL DEPTHOF CUT R. s. HAHN 3,431,685

HIGH-TEMPERATURE ALLOYS Sheet GRINDING WHEEL DEPTH OF CUT FOR 4l50 STEELWHEEL DEPTH OF CUT [FOR T-|5 STEEL INVENTOR' Robert 5. Hahn I I00 200FORCE INTENSITY LB/IN March 11, 1969 R. s. HAHN 3,431,685

GRINDING HIGH-TEMPERATURE ALLOYS Filed Feb. 15, 1965 Sheet 5 of 11 IZO-O O o E z z P WHEEL DEPTH OF CUT U s u g '6 v u I g E 40- -oo [L I uWHEEL WEAR O x X E X X a:

o I T I l 0 $00 200 300 400 500 FORCE INTENSITY LB/IN IZO -|50o IOO- E Em 9 8o- -\ooo u w u. 3 o J r- I 60- u a v u 3 0 L o a 2 o- 4: 3 n:

0 I I 0 I00 200 300 400 500 FORCE \NTENSITY LBS/IN INVENTOR Robert S.Hahn [haul H or eyv,

March 11, 1969 RfSQHAHN 3,431,685

GRINDING HIGH-TEMPERATURE ALLOYS Filed Feb. 15. 1965 Sheet 7 of llACTIVE GRAINS OR CUTTING POINTS INVENTOR.

Robert S. Hahn zlw g March 11, 1969 I R. s HAHN 3,431,685

' GRINDING HIGHTEMPERATURE ALLOYS Filed Feb. 15, 1965 Sheet 6 of 11SLOPE OF WHEEL DEPTH OF CUT CURVE in l|||||||l| I00 200 300 400 500FORCE INTENSITY LB/\N WHEEL DEPTH OFCUT (MI) 0 6 l INVENTOR. Robert S.Hahn March 11, 1969 R. s. HAHN 3,431,685

GRINDING HIGH-TEMPERATURE ALLOYS Filed Feb. 15. 1965 Sheet 9 of 11 FEEDVELOCITY v WORKPIECE WHEELHEAD v CROSS sum:

POSITION '9 INVENTOR. Robert S. Hahn March 11, 1969 Filed IiqZI Feb. 15,1965 Sheet ,0 of 11 F 5 SIZE l v Q 0 CONTACT D l- DRESSING STOP J 003-66 LB/IN 57 LB/IN .ooz- 1% [k [u 2 .J LU LIJ I 3 .00! n 50 LB/IN 22LB/IN *WBRATION DEVELOPED ,NVENTOR O 5 b :5 RObeTt T|ME(SEC) mar I I Honey RADIAL STOCK REMovALUN) March 11, 1969 R. s. HAHN 3,431,585

GRINDING HIGH-TEMPERATURE ALLOYS Filed Feb. 15, 1965 Sheet of 11 .oro- 0A TEST 546 00% p F zo LBs O 0 oo Q0 e p0 n wuss I I 0 I00 200 TIME (55c)"10- i S u 8 4 J H a: z 6

p 0 1 U g u 0 5 IO I5 20 5 FORCE INTENSITY LB/IN a U k B r A 0 0 c 1113.24 FREE PLASTIC SURFACE INVENTOR Robert S. Hahn PLASTIC ZONE UnitedStates Patent Claims ABSTRACT OF THE DISCLOSURE This invention relatesto the grinding of high-temperature metals and, more particularly, to amachine for removing stock from a workpiece by the abrasion process atan unusual rate by operation in a restricted range of force intensities.

In the machining of metal parts, considerable use has been made recentlyof the so-called abrasive machining concept. According to this machiningmethod, the old procedure of performing rough machining operations byuse of single point cutting tools in lathes and planers and millingmachines and then merely finishing or removing a slight amount of thesurface by the abrasive process has been abandoned. This is becausemachine shops have come to realize that, among other things, aconsiderable amount of labor and time is consumed in moving theworkpiece from the conventional machine tool to the abrasive tool inperforming the two operations. It is quite clear that in many instancesthe abrasive process can be used both for the rough cutting and for thefinishing, thus removing the necessity of providing for two operationson two separate machine tools. When one attempts to apply this concept,however, to the new high-temperature alloys, such as T-15, which arecapable of maintaining their strength even at high temperatures, it hasbeen found that with the known machines and methods, it is impossiblenot only to perform abrasive machining but to even, in many cases,perform the finish grinding which gives the product its smooth finish.It has been necessary to machine these alloys, therefore, by veryexpensive and time-consuming alternative methods. These and otherdifficulties experienced with the prior art devices have been obviatedin a novel manner by the present invention.

-It is, therefore, an outstanding object of the invention to provide amethod and apparatus for effectively machining high-temperature alloys.

Another object of this invention is the provision of a method ofremoving stock from workpieces formed of very tough alloys by theabrasive process.

A further object of the present invention is the provision of a grindingmachine for rapidly and accurately machining tough alloys.

It is another object of the instant invention to provide a grindingmachine capable of a machining operation at very high force intensityand with the absorption of large amounts of horsepower.

It is a further object of the invention to provide a grinding machinedesigned to introduce large amounts of power into an abrasive machiningoperation without detriment to the machine itself.

A still further object of this invention is the provision of a methodfor grinding which incorporates a novel discovery in the theory ofgrinding and which permits the removal of large amounts of stock fromvery tough alloys in a minimum amount of time.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

The character of the invention, however, may be best Patented Mar. 11,1969 understood by reference to one of its structural forms asillustrated by the accompanying drawings in which:

FIG. 1 is a perspective view of a grinding machine embodying theprinciples of the present invention;

FIG. 2 is a vertical sectional view of the grinding machine taken on theline II- I|I of FIG. '1;

FIG. 3 is a vertical sectional view of the machine taken on the lineI-IIIII of FIG. 1;

FIG. 4 is a somewhat schematic plan view of the machine;

FIG. 5 is a graph showing wheel depth of cut plotted against forceintensity of grinding for two types of steel using a conventionalgrinding machine;

FIG. 6 shows wheel depth of cut versus force intensity as well as acurve of wheel wear when a force intensity is used which is much greaterthan conventional intensities;

FIG. 7 shows graphs of depth of cut and wheel wear rates versus forceintensity for three conventional grinding wheels;

FIG. 8 is a graph of wheel depth of cut and wheel wear plotted againstforce intensity;

FIG. 9 is a graph of wheel depth of cut and rate of wheel wear plottedagainst force intensity;

FIG. 10 is another graph of wheel depth of cut and rate of wheel wearplotted against force intensity;

FIG. 11 is a graph of depth of cut and wheel wear plotted against forceintensity;

FIG. 12 is a somewhat schematic view of an abrasive wheel surfaceshowing the manner of dispersion of the individual abrasive particles;

FIG. 13 is a somewhat schematic view showing the relationship of agrinding wheel and the surface of the workpiece during grinding;

FIG. .14 is a geometric diagram showing the relationship between anabrasive wheel and the workpiece surface during grinding;

FIGS. 15 and 1-6 are graphs of the slope of the wheel depth of cut curveversus force intensity plus 1 in certain test relationships;

FIG. 17 is a graph of wheel depth of cut versus a certain mathematicalfactor as produced in various tests;

FIG. 18 is a somewhat schematic view of a conventional feed rategrinding machine;

FIG. 19 is a graph showing various positional relationships during theoperation of the conventional machine shown in FIG. 18;

FIG. 20 shows graphically various relationships inherent in the machineshown in FIG. 18;

FIG. 21 is a somewhat schematic view of a grinding machine using thecontrolled force principle of grinding;

FIG. 22 is a graphical representation of certain relationships makinguse of the apparatus shown in FIG. 21;

FIG. 23 is a graphical relationship of wheel depth of cut versus forceintensity making use of the apparatus of FIG. 21; and

PEG. 24 is a somewhat schematic representation of an abrasive particleas it operates to remove stock from a workpiece.

Referring first to FIG. 1, wherein are best shown the general featuresof the invention, the grinding machine, indicated generally by thereference numeral 10, is shown as having a base 11 with a horizontalupper surface 12. Located centrally of the base and overlying thesurface 12 is a turret head 13 adapted to be rotated on occasion about avertical axis. Adjacent the turret head and adapted to be rotated abouta vertical axis spaced from and parallel to the axis of the turret headis a work table 14. The turret head 13 is octagonal in shape and isprovided with eight vertical plane faces 15, 16, 17, 18, 19, 21, 22, and23. On these faces are mounted various machining implements. Mounted onthe face 15 is a wheelhead 24 carrying a vertical spindle 25 on thelower end of which is mounted an abrasive wheel 26. The spindle isdriven by a 40-horsepower electric motor 27 mounted on the face 16.Mounted on the face 17 is a wheelhead 28 carrying a horizontal spindle29 (see FIG. 2), on the outer end of which is carried an abrasive wheel31. Mounted on the face 19 is a wheelhead 32 in which is mounted avertical spindle 33 carrying at its lower end an abrasive wheel 34. A40-horsepower electric motor 35 is mounted on the face 21 and isconnected to the spindle 33 for the driving thereof. Mounted on the face23 is a boring head 36 carrying a boring tool 37.

The electrical and hydraulic controls for the machine are mounted in ahollow abutment 38 extending from the base above the upper surface 12.In FIG. 2, it can be seen that the turret head 13 is fastened to theupper end of a vertical post 39 of cylindrical form which is mounted inthe base 11 for rotation in hydrostatic bearings 41. These bearings areof the well-known type for resisting radial forces and are spaced notonly circumferentially around the post 39 but longitudinally thereof toresist large bending moments in the turret head 13. Within the turrethead is a 40-horsepower electric motor 42 which is connected to thespindle 29 for the driving thereof. Extending into the bottom of thepost 39 is a bore 43 in which is slidably carried an elongated piston 44whose lower end is attached in a fixed portion 45 on the base 11. Theassemblage of the bore 43 and the piston 44 form a hydraulic linearactuator for producing traverse motion as well as reciprocatory motionof the turret head relative to the work table 14.

Extending through the wheelhead 28 concentrically of the spindle 29 is abore 46 in which is slidably carried a piston 47, the piston and borecooperating as a hydraulic linear actuator to produce reciprocation ofthe spindle 29 and the abrasive wheel 31 over the surface of the table14, there being a suitable sliding connection built into the spindle 29to permit it to be continuously connected to the motor 42, despite thereciprocatory motion of the spindle.

FIG. 3 shows another view of the arrangement of the vertical spindle 25,the abrasive wheel 26, and associated equipment. The spindle 25 and thewheel 26 are intended for use with the table 14 in grinding externalsurfaces of revolution, while the spindle 33 and the abrasive wheel 34are intended for grinding internal surfaces of revolution. Similarly,the spindle 29 and the abrasive wheel 31 are provided to produce flatsurfaces.

Referring to FIG. 4, means is shown whereby the turret head 12 may bereciprocated in increments of 45 about its axis relative to the table14. The post 39 is provided with a ring gear 48 which is engaged by arack 49 fastened to the opposite end of a piston rod 51. The piston rodis connected to a piston 52 which is slidably carried in a cylinder 53,the piston 52 and the cylinder 53 forming a hydraulic linear actuatorsuitably controlled to produce reciprocation of the rack and one-wayrotational operation of the turret head 12. This figure also shows thehydrostatic bearings 41 located 120 apart about the post 39.

The operation of the grinding machine will now be readily understood inview of the above description. The operation is performed by rotatingthe turret head 13 until the proper face 15, 16, 17, 18, 19, 21, 22, or23 is located beside the work table 14. This rotation is brought aboutby use of the cylinder 53 operating through the rack 49 and the ringgear 48 to rotate the post 39. Surface grinding is brought about byusing the abrasive wheel 31 and the spindle 29 driven by the40-horsepower motor 42. The wheel is traversed across the work as it isrotated on the table 14 by means of the piston 47 sliding in the bore46. Internal grinding takes place by using the abrasive wheel 34 .on thespindle 33 as rotated by the 40-horsepower motor 35. Traverse motioninto the bore and out again is produced by the post 39 and its bore 43sliding over the piston 44. The slight axial reciprocation of the wheelwhich is used in internal grinding can be produced with this samecylinder using suitable hydraulic valving and controls. When an externalsurface of revolution is to be ground, the abrasive wheel 26 is usedrotated by the spindle 25 which receives its power from the 40-horsepower motor 27. Here again, axial movement of the wheel relative tothe surface is brought about by the sliding of the post 39 over thepiston 44 under the impetus of controlled hydraulic flow to and from thebore 43. It can be seen, therefore, that abrasive operations can becarried on with the present machine using very large force intensitiesbetween the wheel and the workpiece and using very large amounts ofpower without affecting the accuracy of the operation and withoutdetriment to the machine. This is because of the peculiar constructionof a large vertical cantilevered beam that is supported almost entirelyalong its length by the hydrostatic bearings 41. The bending force isapplied to the large post in a fairly short, unsupported portionthereof.

In considering the advantages and the operation of the machine describedabove, it is clear that the value of a machine tool must be evaluated interms of its output versus its cost. In the present case, by takingadvantage of recent developments in grinding technology and making useof the unique properties of hydrostatic bearings, an interesting machinetool configuration has been evolved. The present machine is capable ofrotary surface grinding, O.D. grinding, bore grinding, boring, turning,and facing with single point tools. The workpiece is mounted on a rotarytable, while the surface grinding head as well as the other grindingheads and tooling are mounted on a large turret head which is capable ofvertical feed and of indexing motions as well as rotary feed and rotaryindexing motions. These rotary and axial motions are accomplished bymeans of the large hydrostatic bearings operating on the cylindricalpost of the turret head. The reciproeating motion of the rotary surfacegrinding wheel is accomplished by the axial motion of the head in ahydrostatic sleeve by a feedout motion of the hydrostatic spindleitself. The vertical feed and indexing motion are accomplished by thecylinder shown, while the OD. and ID. grinding are accomplished as hasbeen described. The rotary feed of the turret along with a verticalreciprocating stroke is accomplished by moving the turret post in thelarge hydrostatic bearings. This machine can be applied to variousclasses of workpieces, taking advantage of the economic advantageavailable by the use of abrasive machining. Recent developments inabrasive wheels indicate that metal can be removed quite rapidly byabrasive machining and, in this category, would fall all sorts ofvertical turret lathe work. Cast iron, as well as steel forgings can bemachined by abrasive wheels and, in the present case, single point toolscould be applied, also, when desired on one of the turret head stations.

As an example, a clutch plate which needed to be machined to a 32 rms.finish. It is well known that it is difficult to get decent finishes oncast iron steel and especially, alloy steel by turning. Furthermore, ifan alloy steel is to be machined, the surface speed must be kept low,thus requiring considerable time to generate the surface. By abrasivemachining, a 20 rms. finish can readily be obtained. The volume of metalto be removed from the particular clutch plate was about 1.5 cu. in. Therate of metal removed in grinding often runs about 5 horsepower per cu.in. per min. Assuming a 40-horsepower drive, this gives a metal removalcapability of 8 cu. in. per min. as calculated according to theprocedures set forth in the Tool & Manufacturing Engineer for February1961 at page 111. This means that the 1.5 cu. in. can be removed inabout 11 seconds. If this particular clutch rate were generated by asingle point tool cutting at 400 feet per min. (138 rpm.) on 4150 steelwith .045 radius and .006 lead' (required to give 60 rms. finish), acutting time of 216 seconds is required. Consequently, the clutch platecould be machined in of time with abrasive machining as would berequired by the single point tool. Obviously, as the amount of stock tobe removed is increased, the abrasive machining time will increase and,finally, will equal the single point tool machining time. However, thisbreak even point may often be well above the stock allowances oncastings and forgings. Recent developments have also been made infiberglass reinforced grinding wheels (as described in Grinding andFinishing Magazine for July 1962 at page 40), whereby operation ispermitted at 12,500 feet per min. and forces of 1,000 lbs. for snaggingoperation. This results in high stock removal rates.

By using the cutting tools and abrasive wheels in various combinations,many parts, such as clutch plates, gear blanks, brake drums, and so oncould be machined. Drilling and tapping units could also be incorporatedon one of the turret faces, although it would then be necessary toangularly position the rotary table during the drilling operation.Anti-friction bearing races of the larger sizes can also be ground withthe machine giving excellent concentricity. Furthermore, the machine canbe used for generating general solids of revolution, such asparaboloids, ellipsoids, and so forth. These surfaces can be generatedby numerical control applied simultaneously to the vertical and rotarymovements of the turret.

It is well known that the missile and space industry has encounteredsevere problems in machining high-temperature alloys. Grinding thesematerials has been found to be nearly impossible except by the so-calledhot machining techniques. It would appear to be a distinct advantage ofthe present machine to be able to machine these alloys without the useof hot machining, which is expensive and presents certain problems ofwarpage and so on in the finishing of the machine. In order tounderstand how this may be done, it is necessary to consider some of thediscoveries made as part of the present invention.

In FIG. 5 is plotted the wheel depth of cut against force intensity ingrinding a 4150 steel and also in grinding a high-temperature alloyT-lS. It should be noted that the T-15 curve proceeds along the abscissaaxis and no material was removed. This is an experimental curve obtainedby using a grinding machine with conventional grinding forceintensities. Although, when extremely high force intensities wereattempted, the curves shown in FIG. 6 resulted, wherein the wheel depthof cut and wear are plotted against the force intensity. It can be seenthat, as marked on the graph, a very useful area is obtained when forceintensities between 240 and 280 lbs. per in. are used. When forceintensity was too low, no cutting took place (as was shown in FIG. 5).When the force intensity was too high, of course, the wheel broke down.Obviously, when conventional feed rate reciprocating grinders are used,it is impossible to operate them in this range of force intensity. InFIG. 7 are plotted wheel depth of cut versus force intensity as well aswheel wear rates for three different grinding wheels operating on T-lS,thus indicating that in grinding T-15 with a one inch wide wheel a forceof 400 lbs. and a power requirement of 40-horsepower is necessary.Present grinding machines will not operate in these values. Therefore,by providing a machine particularly adapted for these forces and power,it is possible readily to grind high-temperature alloys, not merely forfinishing these alloys but also for doing the general machining of suchworkpieces.

To properly understand the manner in which the present methods andmachine operate to permit the grinding of high-temperature alloys, itisnecessary to go into the theoretical aspects of grinding in general. Inthis discussion the following nomenclature is used:

=thickness of interference zone =effective grain deflection Kg=eifectiverigidity of grain 1 =grain metal removal parameter (grain depth of cutper unit force) g2l=vo1umetric rate of metal removal N=wheel speed X A=coordinate of A X =coordinate of D a 1 /d-l-h d=effective elasticflattening of wheel and work h=wheel depth of cut I =trochoidal locusfactor:

oar-no (for external) ear car-ea (f t -1)= 2 or in sezrnn m (forsurface) S=ratio of wheel surface speed to work surface speed)\=elfective peripheral grain spacing W=axial width of wheel-workcontact F =normal grinding force F =tangential grinding force IP=horsepower to wheel I-P =horsepower to Work P=force intensity, forceper unit length of contact C=1/a+1/R, the conformity factor e=1/E +1/E2,the elasticity factor E =Modulus of elasticity of wheel E =Modulus ofelasticity of work.

From experimental work on grinding, it has been found that workmaterials may roughly be divided into two categories. In one case alinear behavior of the wheel depth of cut occurs while in the other amore complex situation exists. These cases are described below, and thetechnical factors relating to them are discussed. This work forms thebasis for predicting metal removal rates in abrasive machining ofconventional as well as high-temperature alloys.

Grinding tests have been made, using the controlled force technique. Inthese tests the wheel depth of cut is measured for various normal forceintensities. FIG. 8 shows a typical plot of wheel depth of cut versusforce intensity for a conventional AISI 4150 Steel. The data plotsessentially as a straight line which goes through the origin, withperhaps a little concavity downward due to loading of the wheel. Manymaterials behave in this way. Some materials, on the other hand, havethe characteristic shown in FIG. 9 where the curve emerges from theorigin horizontally and is generally concave upward. It may or may notintersect the Force Intensity axis at some positive value. The cause ofthis behavior is considered to be due to considerable rubbing of theabrasive grain on the workpiece. As the force intensity is increased tohigher values more grains begin to cut causin the curve to be concaveupward. This type of behavior is exemplified by many high-temperaturealloys as illustrated by T-l5.

Since rubbing of the grain can be favored or discouraged by the geometryof the grinding operation, tests were run by grinding with threedifferent conformities, i.e., by grinding on an OD, by internal grindingwith a wheel of moderate size, and with a wheel which nearly filled thehole. FIG. 11 shows the results for AISI 4150 and FIG.

for the high-temperature alloy T-15. It will be noted that on the 4150,where negligible rubbing occurs, the conformity makes little difference.On the other hand, when grinding T-15, there is a very large effect ofconformity. From this we must recognize two categories, one in whichrubbing is essentially absent and one in which considerable rubbingtakes place.

In order to analyze the effects of wheel size, work size and othergeometrical effects, it is necessary to consider the interference zone.The locus of a point on the wheel relative to the workpiece is atrochoidal curve as given previously in The Effect of Wheel-WorkConformity in Precision Grinding, R. S. Hahn, Trans. ASME, vol. 77, No.8, November 1955, 1325-1329. The loci of two peripherally adjacentacting grits I and II, are shown in FIG. 14. Due to deflections of thegrit and the workpiece, grit I does not sweep out the locus O'D.Instead, it sweeps out and generates the surface AB'D'. The interferencezone presented to the grit II is thus Here, the wheel depth of cut isdesignated by h, and the effective elastic flattening of the Wheel-workcombination by a. It is clear that the two parameters d and h areindependent, since in grinding some materials, h may even be zero,although the elastic flattening d may be finite. Under some grindingconditions h is negligibly small compared to d and the interference zonebecomes symmetric.

At any instant there are a number of active grains in contact with theworkpiece as represented in FIG. 12. These grains will have a uniformprobability distribution along their respective interference zones.Therefore, for the purposes of this analysis it is permissible totransfer all active points or grains onto one average interference zoneand to consider it to be uniformly populated with active points.

Next, if one assumes the grain depth of cut to be proportional to theforce K g, there results The instantaneous rate of metal removal Q for mgrains is where b is an effective width of the grains.

The plus sign is used for conventional grinding, the minus sign forclimb grinding.

By solving Equation 1 for g and using the result, the grain depth of cutmay be written as r r,K,

Introducing the grain density 7 (grains/unit/ length) and replacing thesummation in Equation 2 by integration and using Equation 3 givesEquation 4 becomes:

( v :l: XD W(1+I K XA Equation 6 is of the form f(d,h)=0 and relates tothe metal removal.

The grinding force, on the other hand, is given by which is of the formEquations 6 and 7 are independent equations containing d. In principle,d can be eliminated from these giving a relation between F and h.However, by virtue of the assumed linear grain depth of cut relationEquation 3, both Equations 6 and 7 contain the interference zoneintegral. This integral can be immediately eliminated from Equations 6and 7 giving gda:

F gdx which represents the slope of the h vs. force intensity graphs. Itshould be noted that I may possibly be a function of speed and geometry.The quantity (b I is the area swept out per unit force and represents abasic metal removal parameter.

Equation 8 may be rewritten, with the aid of Equation 5 in the followingform different wheels were tested at three different surface speedratios. FIG. 8 shows a typical test curve. The slope of the wheel depthof cut curve (in this case 0.70 10- was measured for each surface speedratio and the results of these tests were summarized. It was seen thatthe slope of the wheel depth of cut curves does increase with surfacespeed and in fact is a linear function of (S+1) as shown in FIG. 15, aspredicted by the theory. Also, the computed values of (b l for eachwheel appeared to be reasonably constant and unaffected by surfacespeed. It is interesting to note that the average value of (b r for theand grit wheels is greater than that for the 46 grit wheels.

From Equation 9 it is seen that the rate of metal removal depends on thetotal horsepower. This is in agreement with data of a commercialgrinding machine company who have found the following horespowerrequirements:

Using the value of (b l of 0.102 10* in Equation 9, one obtains 12.4HP/m. min. which is in reasonable agreement With the commercial data. Itshould be noted that the commercial data refers to large surfacegrinding mlacliines while the present data refers to small grinding w ees.

In the case where considerable rubbing predominates as inferred in FIG.9, the theory presented above does not hold. The data shown on FIG. 10for the OD tests and the ID tests indicates that conformity or thegeometry of the interference zone plays a very important part. Anothertest which reveals the nature of high temperature alloy grinding ispresented in FIG. 16. Here a wheel having a synthetic resin bond wasused to grind T-15. By comparing FIG. 16 with FIG. 10, it is clear thatthe sof more elastic resin bond wheel, cut much slower than the hard,vitreous bonded wheel under these conditions.

The above observations suggest that the contact stress is important. Forthe same force intensity the Polybond wheel does no generate as muchstress as the harder vitreous bonded wheels do. Also, for the same forceintensity, more stress is developed for the OD conformation than for ID.The problem is to discover what quantity will rectify the OD and IDconformity data as well as the data for the Polybond wheel.

The three sets of data on FIG. 10 and the data on FIG. 16 for thePolybond wheel have been replotted on FIG. 17 against the quantity valueof on the present day grinding machines. It can readily be shown thatthe force intensity is related to the Horsepower per unit of wheel faceby 33,000a H s. f.p.m. W

Using this formula, and collecting data on conventional internal,external and large surface grinders, the following table was obtained:

Heald 273A Cincinnati Thompson Internal 6" x 30 center 5 it. x 14 ft.

type surface grinder Wheel width (in.) 1. 2% 6 Wheel diameter 1. 24 24Work diameter -3 +1. 5 00 HP 5 7. 5 40 0.-.. 67 1. 41 083 E 158 X158X10- 158 10-' PM, 13. 8 7. 8 18. 4

Panz (O/EWL. 105X10 066x10 057x10 From this, the Heald Internal has thecapability of producing the highest stress-width factor of the group.However, the magnitude of these factors, as found on the above machines,are completely inadequate for abrasive machining high-temperaturealloys. These values all fall very close to the origin on FIG. 17.

The following discussion is also olfered for an understanding of thereasons for the unobvious results obtained and advantages available bythe use of the methods of the invention.

The difficulty of machining high temperature and exotic alloys is wellknown. The development of techniques to machine these alloys efiicientlywill undoubtedly reduce manufacturing costs of many space and militarydevices. Recent research and development work in connection with thegrinding or abrasive machining of these materials indicates thatconsiderable improvement can be made in processing these materials, by anew technique called Controlled Force Abrasive Machining. The type ofmachining operations to be considered are those of turning, boring, andthe facing of solids of revolution such as are commonly carried out onvertical turret lathes. Parts with simple as well as fairly complexcross sections consisting of grooves, shoulders, and contours such asjet engine par-ts, are to be considered. Drilling and tapping operationsare to be excluded. On difficult-to-machine materials, where the cuttingspeed is low and tool life short, the abrasive machining of these partsbecomes very advantageous. However, excessive residual stress andthermal damage to the workpiece must be avoided. This appears to bepossible by Controlled Force Abrasive Machining at high work speeds willbe discussed.

Before discussing controlled force abrasive machining it is important tounderstand certain characteristics of the conventional feed rategrinding process. In present grinding machines it is customary to feedthe wheel into the work at a given feed rate V and then to provide adwell or spark-out time. FIG. 18 illustrates the basic elements of aninternal grinder where the spindle is relatively flexible. Although theconsiderations to follow relate to internal grinding, they apply equallywell to external and rotary surface grinding since some flexibility isalways present. The line OD in FIG. 19 illustrates the cross slideposition during feed and DE during the dwell. The line OCF representsthe instantaneous position of the grinding wheel itself, which differsfrom the cross slide position because of elastic deflections. Theinstantaneous grinding force induced between wheel and work isproportional to the vertical difference between the two curves in FIG.19. It has been shown (in an article entitled Controlled Force GrindingANew Technique For Precision Internal Grinding by R. S. Hahn, presentedbefore the ASME Winter Meeting, November 17 through 22, 1963) that thisforce is given by where:

F,,=normal grinding force I=metal removal parameter i.e. the wheel depthof cut per unit normal force n=work speed K=system static stiffness,includes flexibility of work support, bearings, etc.

Mi=cross slide mass h =wheel depth of cut n=ratio of tangential force tonormal force.

Here V/In is the steady state grinding force and 1/ IKn is the timeconstant of the wheel-work-machine system. From Equation A it can beseen that the instantaneous grinding force depends upon the feed rate V,the metal removal parameter I pertaining to the wheel-Work combination,and the effective rigidity K of the machine tool.

The grinding action of an abrasive wheel depends directly on thegrinding force. If the force is small, the wheel tends to dull, if toohigh, the wheel breaks down at an excessive rate. FIG. 20 shows wheelwear vs. time for several levels of force intensity (force per unitwidth of wheel-work contact). At 22 lb./in. the wheel is not wearing andbecomes dull and glazed. At 66 lb./in. wheel wear is excessive.

In view of the above and the fact that the grinding force depends on thefeed rate V, the metal removal parameter I and the static machine toolstiffness K, it is easy to understand why grinding is an art and how amachine operator has to manipulate grinding conditions untilsatisfactory results occur. On difficult to grind materials he may neverattain these conditions. Moreover, a further variation in grinding forceoccurs as a result of stock variations from piece to piece.Consequently, it is clear that in feed rate grinding, the grinding forceand thus the grinding action are complicated functions of severalvariables.

The recently developed controlled force method of grinding combined withthe present invention eliminates these ditficulties. In this method thewheel is pressed against the workpiece with a prescribed force and thefeeding velocity of the cross slide becomes the dependent variable. FIG.21 shows the controlled force method applied to internal grinding. Thewheel head is mounted on a frictionless cross slide of mass M. A force Fis applied to the frictionless cross slide which causes a reaction Fbetween the wheel and workpiece. The wheel then grinds under this forceaccording to its ability to remove stock. The progressive forwarddisplacement of the cross slide indicates the stock removal. Thewheelhead may be swivelled to compensate for angular deflection. Thecross slide is retracted to the dressing stop D for wheel dressing orunloading the workpiece. The dressing stop and size contact are movedperiodically to compensate for wheel wear.

The cross slide mass M and the spindle spring constant K constitute avibratory system. Although FIG. 21 depicts a flexible internal grindingspindle, K represents the overall system static stiffness and thusaccommodates flexibility in the work supporting device and the machinein general. Moreover, this same system in effect, applies to external,and rotary surface grinding as well as internal grinding.

The stability of this K-M vibratory system has been studied and it hasbeen shown that, if the work rotational speed is above a certaincritical value, a rounding up action will take place, i.e., an initiallyout of round workpiece will become round and consequently controlledforce grinding is possible. At low workspeeds the system is unstable andviolentvibrations may build up.

From the above it can be seen that controlled force grinding resultsinefficient stock removal, control of the self-dressing effect and wheelsharpness, and a simplification of the grinding process. The grindingforce is no longer dependent on many variables.

The metal removal characteristics of grinding wheels can be readilymeasured using the controlled force method. FIG. 22 shows a typicalresult where the radial stock removed is plotted against time. The slopeof the curve is the rate of radial advance. The wheel depth of cut orradial advance per revolution of the work is obtained by dividing therate of radial advance by the work speed.

Three types of abrasive action have been observed. FIG. 23 shows wheeldepth of cut vs. force intensity. Under the conditions of the test onlyrubbing occurred below 4 lb./in. From 4 to about 16 lb./in. ploughingoccurred and above 16 lb./ in. cutting occurred. The ploughing processis one where the abrasive grain plastically plows a groove with themetal extruding sideways into ridges along the side of the groove. Thesehighly extruded ridges may be removed by other grains or fragments maybreak off. In ploughing no chip is formed ahead of the grain. In thecutting process chips actually form ahead of the abrasive grain in theusual way.

The key to understanding the grinding behavior of many high temperaturealloys is best explained in terms of the free plastic surface. (See thebooklet entitled On The Nature of the Grinding Process by R. S. Hahn inthe Proceedings of the Third International Machine Tool and ResearchConference at the University of Birmingham, England, September 1962,published by the Permagon Press.) In FIG. 24 a plastically deformed zoneis illustrated under the leading edge of the abrasive grain. If afracture of the free plastic surface occurs before the grain reaches thepoint 0, a chip will be formed and flow up the rake surface 'of thegrain, otherwise only ploughing or rubbing will take place. Thisaccounts for the tremendous difference in grindability of variousmaterials. FIG. 5 shows the wheel depth of cut curves for AISI 4150 anda high vanadium high speed steel T-l5 both heat treated to the samehardness. The tremendous difference in grindability is obvious. Ingrinding the A181 4150 sample, the grain forms a chip readily and littleor no rubbing or ploughing takes places, whereas with T15 many of thegrains rub or plough until very high force intensities are reached.

In general, then two extremes of grinding behavior occurs. When littleor no rubbing or ploughing takes place, the wheel depth of cut curve isa straight line through the origin as shown in FIG. 8. When the workmaterial refuses to fracture in the free plastic surface excessiverubbing takes place and the wheel depth of cut curve emerges from theorigin in a horizontal direction like a parabola and only at high forceintensities does it turn upward as shown in FIG. '9'. Curves for Rene41, Inconel X, 1 77PH, Ther-mold I have been determined and lieintermediate to those in FIG. 8 and FIG. 9.

The significance of controlling the force intensity becomes apparentfrom FIG. 6, which shows that there is only a narrow range of forceintensity within which successful operation can be achieved. If theforce intensity is too low, no metal removal takes place; if it is toohigh, the wheel wear rate is excessive. From this it is easy tounderstand how the grinder operator, using the conventional feed ratemethod, must cut and try many combinations and even then may not findsuccessful conditions. Furthermore, the force intensities required forgrinding some of the high temperature alloys lie outside thecapabilities of present day grinding machines. FIG. 6 also shows thecurve for normal tool steels, i.e., A181 4150 and the approximate limitof conventional grinders. Since the maximum attainable force intensity Pis related to the wheelhead horsepower by Pm s.f.p.m. W

it is easy to show that P lies between 10 and 20 lb./in. for mostcommercial grinding machines. Consequently, it is practically impossibleto grind T-15 at a hardness of Rockwell C= on any conventionalequipment, as people have found.

Much remains to be learned about thermal damage and residual stress inhigh temperature alloys. However, it is apparent that thermal damage tothe workpiece can be minimized by operating at high workspeed. Theelimination of heat checks and considerably improved tool life have beenobserved in carbide tools sharpened at 600 f.p,m. workspeed. (See Trans.ASME, May 1956, The Relation Between Grinding Conditions and ThermalDamage in the Workpiece by R. S. Hahn.) It is clear that residualstresses can be kept within reasonable limits by using high workspeedsand force intensities which keep the Wheel in a sharp condition.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to include all such as properlycome within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent 1s:

1. A grinding machine for a workpiece formed of a high-temperaturealloy, comprising (a) means including a base and a workholder mounted inthe base for supporting the workpiece,

(b) means including a wheelhead and an actuator for advancing a rotatingabrasive wheel into contact with the workpiece to perform astock-removal operation,

(c) a control operating on the actuator for maintaining the forceintensity between the wheel and the 13 14 workpiece in the range from200 to 400 lbs/inch 0f 5. A grinding machine as recited in claim 1,wherein wheel, a hydraulic cylinder is operative along the axis of the(d) a vertical supporting post having a head which is post to move itlongitudinally in the said bearing means.

fixed to the post which head carries the wheelhead and the abrasivewheel, and 5 References Cited (e) bearntgh meantsmgulntecihintheflllaase fand egtiznd iinag1 UNITED STATES PATENTS aroune pos an eng wise ereo a su" s an mm it??? 1122:; are; 2. A grindingmachine as recited in claim 1, wherein 3052067 8/1962 Dilks 51 35 thehead is a turret on which is mounted a vertical spindle 1o 31979218/1965 H h] X for operation on surfaces of revolution and a horizontal 0er spindle for operation on fiat surfaces. OTHER REFERENCES 3. Agrinding machine as recited in claim 2, wherein the workholdersupporting the workpiece is a rotatable table whose axis is spaced fromand parallel to the axis of the post, and wherein means is provided toindex the Robert Hahn: On the Nature of the Grinding Proc- 15 ess,Proceedings at the Third International Machines Tool 2nd ResearchConference, September 1962, pp.

post and turret head to bring either spindle into registry 129454- wlththe table. JAMES L. JONES, Primary Examiner.

4. A grinding machine as recited in claim 1, wherein the bearing meansconsists of a set of hydrostatic bear- 20 US. Cl. X.R.

ings. 51-35

